During the summer of 1999, a concrete contractor placed an exterior parking lot for a commercial building in the Midwest. The concrete was air entrained and received a broomed finish, followed by 3 days of moist curing with plastic-coated burlap. Because some of the concrete set at a different rate than the surrounding concrete, the appearance of the broomed finish wasn't uniform and "soft surface conditions" were reportedly exhibited. The parking lot owner questioned the quality of the concrete, and had cores sent to a test lab. The petrographic report stated that the coarse aggregates were generally sound but the entrained-air content in the top 1/4 inch of the two samples was inadequate to resist freeze/thaw damage. The petrographer further concluded that freeze/thaw damage was expected to be a problem for the concrete slab.
The number of construction projects using wet-mix instead of dry-mix shotcrete continues to grow, primarily because wet-mix shotcreting can achieve a higher production rate. Contractors using this concrete placing method have a number of pump options. But what's the best pump type to own or rent? The answer depends mainly on the volume of concrete to be placed.
In structures for which rebar corrosion isn't an issue, calcium chloride is the most economical and effective accelerating admixture. It's widely used in a liquid solution in unreinforced pavements, residential foundation walls, and floors placed in cold weather. It helps concrete set and gain strength faster, allowing finishers to complete their work sooner and decreasing the likelihood of damage caused by early freezing.
Concrete paving contractors are placing more and more overlays--a repair option once dominated by the asphalt paving industry. Of the record 26 winners and finalists in 12 categories in the 2000 National Awards Program for Excellence in Concrete Pavement, more than one-fourth of the projects involved whitetopping of asphalt or concrete pavements. Sponsored by the American Concrete Pavement Association and Concrete Construction, the awards recognize contractors, engineers, and owners who completed outstanding projects in 1999.
Steel dowels, when aligned carefully and debonded from the concrete, help to carry loads across joints in concrete slabs even after the joints have opened too much to prevent load transfer by aggregate interlock. However, traditional round dowels may cause slab cracking by restraining movement along doweled longitudinal joints when workers place large floor slabs in long, alternating strips, with infill strips placed later. When the infill concrete tries to shrink, the dowels prevent slab movement parallel to the longitudinal joint, causing internal stresses.
